Generally, the output torque of an ordinary screwdriver is not adjustable, relying merely on the user's experience to control the torque applied. So when the torque needs to be controlled precisely, as in the case of interconnections between various parts of precise instruments, or in the case of brittle or easily damaged fastening and/or fastened parts, excessive force or too large an exerted torque may damage the parts. In such a case, experience of the operator is the sole reliance, and therefore there is a certain prerequisite for the user, for example, he/she needs to be a skilled mechanic.